1. Field of the Invention
The present invention relates to a process for producing epichlorohydrin useful as, for example, a solvent in various fields, a raw material for the production of epoxy resin and synthetic rubber, and a stabilizer for chlorinated rubber.
2. Description of the Related Art
Hitherto, epichlorohydrin has been typically produced, as shown in the following reaction equations (1), (2), and (3), by the subsequent steps: allyl chloride synthesis step (1) from the chlorination reaction of propylene, dichloropropanol synthesis step (2) from the chlorohydrination reaction of the allyl chloride, and epichlorohydrin synthesis step (3) from the saponification reaction of the dichloropropanol. ##STR1##
However, the above-mentioned production process of epichlorohydrin has serious problems from the practial point of view as follows:
(A) The first step (1) is generally carried out in the absence of a catalyst in a vapor phase. However, this step (1) has disadvantages, especially from the industrial standpoint, that (i) since the reaction temperature is high, the yield of the desired product is low due to the formation of various kinds of by-products, (ii) the clogging of the reactor caused by the by-produced polymers occurs due to the carbonization of the by-produced polymers formed upon contact of the propylene with chlorine and also clogging of the heat exchanger is caused by the deposition of the by-produced polymers when quenching the gaseous reaction product by a solvent, and (iii) the equipment is severely corroded due to the handling of hydrogen chloride at an elevated temperature.
Furthermore, in the above-mentioned step (2), since the solubility of the allyl chloride in water is low, the oil phase is formed when the concentration of the allyl chloride is intended to be increased. When the reaction is carried out in this condition, the chlorine is dissolved in the oil phase and the addition reaction of the chlorine to the allyl chloride proceeds in preference to the desired reaction of the allyl chloride with water. As a result, the side reaction of forming trichloropropane is increased. In order to suppress this side reaction, the reaction should be carried out at a low concentration of allyl chloride, which, however, results in the production of the desired dichloropropanol as a low concentration solution thereof.
When the dichloropropanol is obtained at a low concentration, an excessive amount of energy is required in the above-mentioned step (3). That is, when the desired epichlorohydrin is stripped with steam while the saponification of the reaction equation (3) is carried out, extra energy (i.e., heat energy corresponding to the sensible heat) is required for increasing the temperature of the low concentration solution of dichloropropanol and an excessive amount of steam is required for stripping the desired epichlorohydrin due to the low concentration thereof.
Furthermore, since the dichloropropanol formed in the above-mentioned reaction equation (2) is obtained in the form of a mixture of two isomers (i.e., 1,3-dichloro product and 2,3-dichloro product), the saponification conditions of both isomers cannot be optimized at the same time because the saponification reaction rates of the isomers are extremely different. As a result, the improvement in the yield of the desired epichlorohydrin is naturally limited. In addition, in the above-mentioned conventional process, a large amount of lime is unpreferably consumed for neutralizing hydrogen chloride formed in an equimolecular amount in the reaction equation (2).
Various attempts have been proposed to improve the above-mentioned problems or disadvantages. For example, the process represented by the following reaction equation (4) to (7) is disclosed in Khim. Prom. No. 6, 328-335 (1982). ##STR2##
However, these reactions still have various problems or disadvantages from the practical point of view, although this process is advantageous in that the reaction can be carried out at a higher concentration when compared to the above-mentioned case. For example, in the above-mentioned chlororation reaction step (5), the CoCl.sub.2 catalyst must be separated after the reaction. In addition, the yield of the desired product is not high (e.g., 47.5% to 84.2%) and, therefore, the unreacted allyl acetate should be separated and recovered. Furthermore, side reactions such as a substitution reaction with chlorine occurs and, accordingly, the methyl group of the acetyl group is, for example, chlorinated, so that the acetic acid is wastefully consumed and allyl chloride is formed as a by-product. The resultant chlorinated product is obtained as a mixture of the 2,3-dichloro isomer and 1,3-dichloro isomer and, therefore, the yield of the desired product in the saponification step is naturally limited. Furthermore, although the organic solvent used in the reaction (5) must be recovered, a portion of the organic solvent is inevitably lost during the distillation.
In the above-mentioned reaction step (6), the reaction is an equilibrium reaction and, therefore, the H.sub.2 O ratio of the dichloropropyl acetate to the dichloropropanol should be increased to increase the conversion rate. However, since the starting dichloropropyl acetate has the highest boiling point in the equilibrium system, all the reaction mixture including water, as well as the reaction products such as acetic acid and dichloro propanol, must be vaporized to recover the unreacted dichloropropyl acetate for the purpose of recycling. This means that a large amount of heat energy is required for increasing the conversion of the reaction (6). Furthermore, although a by-product obtained by chlorinating the acetyl group in the chlorination reaction (5) is converted to monochloro acetic acid in the hydrolysis reaction (6), the separation thereof from the desired 2,3-dichloro-1-propanol is difficult because the boiling point of monochloro acetic acid is 187.degree. C., which is close to that of 2,3-dichloro-1-propanol.
In the above-mentioned saponification reaction (7), the optimization of the reaction conditions is difficult due to the difference in the reaction rates between the 2,3-dichloro isomer and the 1,3-dichloro isomer as mentioned above.
Another proposal to improve the problems or disadvantages of the above-mentioned typical conventional case is represented by the following reaction equations (8) to (10) as disclosed in Compend. - Dtsch. Geo. Mineraloelwiss. Kohlchem. p. 318-326 (1975). ##STR3##
This process is advantageous in that the selectivities of the equations (8) and (9) are about 90% and about 96%, respectively and, accordingly, the selectivity of allyl chloride from propylene is about 86.4%, which is more than 10% higher than that of the conventional process. However, this process still has the following disadvantages:
(i) The reaction (9) should be carried out in the presence of CuCl or FeCl.sub.2 catalyst in a non-aqueous system to prevent hydrolysis. Accordingly, the water must be removed from the reaction mixture of the previous reactions (8), which generates the water and is generally carried out in the presence of water.
(ii) The reaction (9) also requires the use of expensive anhydrous hydrogen chloride.
(iii) In order to separate and recover the catalyst from the reaction mixture, the unreacted allyl acetate, the resultant acetic acid, the solvent, and the like should be removed by distillation.
(iv) Since the reaction subsequent to the equation (10) is the same as in the above-mentioned conventional processes, the above-mentioned problems and disadvantages are still involved in this process.